1. Field of the Invention
The present invention relates to nuclear fuel pellets and a fabrication method thereof, and more particularly, to nuclear fuel pellets having large grains containing additives and fast creep deformation, and a fabrication method thereof.
2. Description of the Related Art
Nuclear fuel, one of the most significant elements used in nuclear reactors, generates energy by maintaining a nuclear fission chain reaction. Nuclear fuel must be fabricated such that, while it is in use, its mechanical soundness with a cladding tube covering (or surrounding) it is maintained and a nuclear fission product or the like is not leaked out, as is possible. A UO2 sintered pellet used as nuclear fuel is charged in a cladding tube (e.g., a zirconium alloy cladding tube) so as to be used in a hermetically sealed state. The UO2 sintered pellet is fabricated by forming a green pellet generally by using UO2 powder as raw material powder and then sintering the green pellet at 1,600° C. to 1,800° C. for two to eight hours in a hydrogen gas atmosphere. The UO2 sintered pellet fabricated thusly has a density of about 95% TD and a grain size ranging from about 6 μm to 10 μm.
Recently, in order to enhance the economical efficiency of nuclear fuel and to reduce the amount of spent fuel, it is necessary to extend the fuel discharged burn-up. Research on fuel pellets focuses on the high burnup fuel pellets. However, in this case, the high burnup leads to an increase in the amount of fission gases such as xenon (Xe) , krypton (Kr). The fission gases in the pellet are continuously released out from the pellets during the reactor operation and increase the internal pressure of nuclear fuel rod. The increased internal pressure induced by the nuclear fission gases increase the stress working on the cladding tube, resultantly reduces the safety margin of the nuclear fuel rod. Thus, in order to solve such a problem, the nuclear fission gases generated due to nuclear fission must be released out as small amounts as possible to the outside of the sintered pellet (i.e., the pellet).
The process of nuclear fission gas release to the outside of the nuclear fuel pellet is generally known as follows. The nuclear fission gas is generated in a grain, moves to the grain boundary through diffusion so as to exist as bubbles in the grain boundary, and when the bubbles are increased to reach a certain amount, a bubble tunnel is formed along the grain boundary, and the bubbles are then discharged to the outside of the nuclear fuel pellet. Accordingly, if the grain size of the pellet is increased, the distance for the nuclear fission gas to reach the grain boundary is increased, allowing it to stay in the sintered pellet for a longer period of time, and as a result, fission gas release from the pellet can be reduced. Accordingly, a nuclear fuel pellet designed for a high burnup fuel requires for an increase in its grain size. In order to increase the grain size of the sintered pellet in the process of fabricating a nuclear fuel pellet, various additive elements such as Al, Cr, Ti, Nb, and the like, as widely known, may be used. The content of the additives to uranium cations is about a few parts per million (ppm) to thousands of ppm in weight, and the amount of additives added differs depending on their types.
The UO2 nuclear fuel pellet is charged in a zirconium alloy cladding tube and burned up in a nuclear reactor. During irradiation, the nuclear fuel cladding tube is deformed inwardly while the sintered pellet expands outwardly due to a swelling phenomenon as a result of neutron irradiation, so the sintered pellet and the cladding tube are brought into contact to generate stress. This interaction between the nuclear fuel and the cladding tube is called a pellet-clad interaction, and continuation of the interaction can end in possible damage to the cladding tube. If the cladding tube is failed during reactor operation, radioactive material would flow out of the cladding tube, reducing the safety margins of the nuclear reactor.
There is a high possibility that a nuclear fuel is used in an extreme situation such as in a reactor power up-rate or frequent transition operation or the like. In particular, in the case of a highly irradiated nuclear fuel, when the reactor power increases for a short time period, the temperature of the nuclear fuel pellet rises to press the cladding tube due to thermal expansion, and when a great deal of stress is applied to the highly irradiated cladding tube for a short time period, the cladding tube would be possibly failed. Thus, in order to effectively reduce the pressure applied to the cladding tube due to the thermal expansion of the nuclear fuel pellet resulting from the reactor powder change, an improvement of the pellet softness is required by developing a pellet with a large amount of initial deformation and a large creep deformation rate (i.e., speed).